SALT LAKE CITY--Bacteria are masters of disguise: Their ability to put on a variety of faces helps them evade immune responses and thwart the best efforts of drug designers. For years, researchers assumed that their hosts' immune systems had spurred the evolution of this flexibility. But that might not always be the case. New research shows that Salmonella bacteria might have developed their ability to change to help foil amoebae that are out to devour them.

Salmonella enterica causes food poisoning in a number of animals, including humans, cows, and pigs. These hosts' immune systems identify the bacteria by the sugars coating their cell membranes. The bacteria can change their coats--and this would enable them to reinfect an animal previously exposed to Salmonella. But they don't. Puzzlingly, bacteria with the same coat can easily reinfect the same animal. A possible explanation, reasoned evolutionary biologist Jeffrey Lawrence of the University of Pittsburgh, is that natural Salmonella predators in the animals' guts, such as amoebae, exert a stronger pressure than the immune system exerts.

To test this idea, Lawrence and colleagues lined up two feasts for predatory amoeba in a petri dish: rows of two Salmonella strains with different sugar coatings. The amoeba devoured one strain faster than the other. When tested with seven different Salmonella strains, the amoeba ate the tastiest Salmonella 2.5 times faster than the least attractive one, Lawrence reported here on 21 May at the general meeting of the American Society for Microbiology. Apparently the sugar coat makes all the difference.

Next, Lawrence's team isolated amoeboid predators from frogs, insects, and other animals and discovered that each preferred a very specific type of sugar coating on their Salmonella. That suggests that the bugs might present the least tasty option in order to survive among the predators in any particular host, Lawrence says. If this turns out to be true, he says, the most important evolutionary pressure on Salmonella might not come from the host but from the amoeba species it carries.

Geneticist John Roth of the University of Utah in Salt Lake City calls the work "really cool." It shows that when studying infectious agents, researchers shouldn't just think about their interactions with humans. "We need to think about these bacteria as they really live, in soil and all over," Roth says.